Also you can use Power and Precision V4 Application for this purpose.

You know all the universities, correct?  If you could use cluster sampling, you might check out a book such as Cochran, W.G(1977), Sampling Techniques, 3rd ed., John Wiley & Sons, if you are doing a quantitative study. 

You might research "area sampling." 

Hello Mariya

Please find the attached links which may assist you to answer your question.

Some of these links are articles or handouts and others are previous similar questions that were answered in researchgate

I hope this could help you

Best wishes

Mansour

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2876926/

http://sphweb.bumc.bu.edu/otlt/mph-modules/bs/bs704_power/BS704_Power_print.html

http://www.vanbelle.org/chapters%5Cwebchapter2.pdf

https://www.researchgate.net/post/How_can_we_determine_the_sample_size_from_an_unknown_population

https://www.researchgate.net/post/What_is_the_best_method_to_determine_the_sample_size_when_the_total_population_is_unknown2

https://www.researchgate.net/post/How_can_we_determine_the_sample_size_for_an_unknown_population

https://www.researchgate.net/post/Any_advice_on_sample_size_for_unknown_population_for_quantitative_methods

https://www.researchgate.net/post/In_an_unknown_or_infinite_population_how_can_I_list_my_sampling_frame

Conference Paper Sample Size Determination for Non-Finite Population

Google GPower and you will get a free and powerful app to put in practice the advices gave!

Best wishes!

Thank you so much everyone for your help. I apologise for the delayed response. I'll have a look at eachurch suggestion more closely. Best!

Dear RG colleague,

Determining the sample sizes involve resource and statistical issues. Usually, researchers regard 100 participants as the minimum sample size when the population is large. However, In most studies the sample size is determined effectively by two factors: (1) the nature of data analysis proposed and (2) estimated response rate.

For example, if you plan to use a linear regression a sample size of 50+ 8K is required, where K is the number of predictors. Some researchers believes it is desirable to have at least 10 respondents for each item being tested in a factor analysis, Further, up to 300 responses is not unusual for Likert scale development according to other researchers.

Another method of calculating the required sample size is using the Power and Sample size program (www.power-analysis.com).

Regards,

In order to answer this question/problem, several remarks have to be studied.

1. General remarks:

Research studies are usually carried out on sample of subjects rather than whole populations. The most challenging aspect of fieldwork is drawing a random sample from the target population to which the results of the study would be generalized. The key to a good sample is that it has to be typical of the population from which it is drawn. When the information from a sample is not typical of that in the population in a systematic way, we say that error has occurred. In actual practice, the task is so difficult that several types of errors, i.e. sampling error, non-sampling error, Response error, Processing error,…

In addition, the most important error is the Sampling error, which is statistically defined as the error caused by observing a sample instead of the whole population. The underlying principle that must be followed if we are to have any hope of making inferences from a sample to a population is that the sample be representative of that population. A key way of achieving this is through the use of “randomization”. There several types of random samples, Some of which are: Simple Random Sampling, Stratified Random Sampling, Double-stage Random Sampling... Moreover, the most important sample is the simple random sample which is a sample selected in such a way that every possible sample of the same size is equally likely to be chosen. In order to reduce the sampling error, the simple random sample technique and a large sample size have to be developed.

2. Specific remarks:

The following factors are highly affected the sample size and need to be identified:

• Population Size,
• Margin of Error,

· Confidence Level (level of significance) and

• Standard of Deviation.

Then, the sample size can be estimated by,

Necessary Sample Size = (z-score or t-value)2 * StdDev*(1-StdDev) / (margin of error)2 .

This is a good question to think about, but it probably cannot be answered because the answer requires more information that you have available. Your sample size for a survey is based on the question with the greatest variability in the sampled population.

1) You could do a pilot study, but that takes resources that might be better spent on the primary study.

2) I would look in the published literature at the most recent 50 published papers. What sample size did they plan on using and what sample size did they achieve? Find the median number of surveys. Then I would look at my budget and figure out if I have a chance of getting that number of surveys. I would plan on some faculty refusing to take the survey, and I would keep track of these people. The rejection rate is one important metric for survey performance (I would also keep track of people who agree, and those that need some sort of encouragement or reminder). Within budget, try to get as many surveys as possible. If necessary, consider focusing the project: only people in the College of Agriculture, or only faculty in the entomology and physics departments, or only female faculty, or only _________.

This formula is the easiest way to do it.

1- Estimating Sample Size based on proportion

2- Estimating Sample Size based on mean

http://documents.routledge-interactive.s3.amazonaws.com/9780415628129/Chapter%209%20-%20Determining%20sample%20size%20final_edited.pdf

Also you can use Power and Precision V4 Application for this purpose.